This invention relates to a sealed storage battery and a method for making its electrode, and more particularly, the present invention relates to an alkaline storage battery.
Nowadays, a lead-acid storage battery and a nickel-cadmium storage battery are predominantly used as a sealed secondary battery. Although the lead-acid storage battery is of low cost, this battery is insufficient from the point of the view of weight density or cycling life-time when used as the electric power supply of a portable apparatus which is in use for a long time. On the other hand, although the nickel-cadmium storage battery is of comparatively high cost, the demand for this battery has been increasing, and more particularly, this battery has become widely applied in fields where high reliability is required because this battery can remove the drawbacks of the lead-acid storage battery. Various improvements for providing nickel-cadmium storage battery with high capacitance have been made since it is desired to have higher energy density electric power supplies of the portable apparatus in addition to the special merits described above. However, a cadmium electrode as a negative electrode has a high utilization factor of an active material, especially a large electrode decrease at a high-rate discharge in comparison with a nickel electrode as the positive electrode. Moreover, when constructing the storage battery of capacitance regulation at the positive electrode, it is necessary to take allowance to except capacitance at the positive electrode so as not to generate hydrogen at the negative electrode when overcharging. Therefore, the negative electrode has the capacitance capable of charge and discharge larger than the positive electrode. Moreover, it is necessary for the porosity of the negative electrode to be increased to an optimum value in order that an overcharge, oxygen gas generated from the positive electrode is efficiently absorbed by the negative electrode. However, it is not possible for the negative electrode to have higher energy density until a rapid improvement is made. Therefore, there is a limit in the improvement of the energy density of the nickel-cadmium storage battery.
Recently, a metallic oxide-hydrogen storage battery has attracted the attention of many people in which battery a hydrogen-occlusion-alloy or hydrogen-storage-alloy, which can occlude and release electrochemically hydrogen is applied as the material of the negative electrode, instead of the nickel-cadmium storage battery. In this storage battery, even if hydrogen is generated from the negative electrode on overcharge, hydrogen is dissipated through discharge or is occluded by the hydrogen-occlusion-alloy of the negative electrode, in so far as hydrogen is not released out of the battery. Moreover, this storage battery has a higher energy density per unit volume than the nickel-cadmium storage battery. As a result, where the capacity of this storage battery is equal to that of te nickel-cadmium storage battery, this storage battery can be constructed such that the volume of the negative electrode of this storage battery is smaller than that of the cadmium negative electrode. Therefore, a larger active material as the positive electrode can be appropriated in the residual space of this storage battery, so that a higher energy density is expected. Moreover, the material of the hydrogen-occlusion-alloy obviates the need for a metallic cadmium which is a main component of the cadmium electrode, so that there is little polution by heavy metal or the like.
With these points as background, a new secondary battery is developed for use in various fields. However, many problems need to be solved if this storage battery is commercialized. One of the important problems is that techniques for constructing a sealed storage battery by means of a simple method and for lenghthening the life-time of the battery must be carried out to be competitive with the nickel-cadmium storage battery. Namely, metallic oxide-hydrogen storage battery, whose maintenance is easy and which has high reliability, must be perfected. From the above-mentioned stand point, as the condition which are required for the hydrogen-occlusion-electrode to use the hydrogen-occlusion-alloy, the hydrogen-occlusion-electrode must have stability in an alkaline electrolyte and occlude and release electrochemically hydrogen. Moreover, other important conditions are the following (1)-(3).
(1) The hydrogen-occlusion-electrode must have a large capacity for occluding and releasing electrochemically hydrogen, and discharge capacitance must not decrease even if charge and discharge are repeated.
(2) An ordinary sealed storage battery is designed such that for safety, the pressure gas in the battery is kept at a given value or below with a safety valve being operated when the pressure in the battery becomes 10 to 15 kg/cm.sup.2. Therefore, the material must be selected which does not generate hydrogen on charging within given temperature ranges where the battery is used since in this storage battery system, it must designed designed as well as the ordinary sealed storage battery as described above.
(3) The hydrogen-occlusion-electrode must have a corrosion resistance to oxygen generated from the positive electrode on overcharging. Moreover, the alloy material must be used which has a function for smoothly carrying-out oxygen-elimination reactions that on the surface of the alloy where, the reaction of oxygen with occluded hydrogen gives water. Moreover, an alloy material should be used to construct the electrode. Although many alloy materials and many methods for constructing an electrode satisfying these conditions (1) to (3) have been pursued, they do not always reach the operating level of the sealed nickel-cadmium storage battery, i.e., where the cycling life-time is 500 cycles or above, and the inner pressure of the battery is 5 to 10 kg/cm.sup.2 or below, when the battery is charged to the level of approximately 1/3 CmA.
Concerning (1), an alloy of AB5, which is a basic form of CaCu.sub.5 type alloy and in which lanthanum series metals are mainly used as A and nickel is mainly used as B, has the largest possibility of a practical application. However, even if the alloy of AB.sub.5 is used as a hydrogen-occlusion-electrode, the electrode is oxidized by oxygen gas generated at the positive electrode on overcharge to decrease the capacity of occluding and releasing hydrogen. In order to solve the above-mentioned problem, a method is used so that the surface of the alloy is covered with a metal having the property of corrosion resistance (Japanese patent provisional publication No. 61-64069 or No. 61-101957) and the entire electrode is covered with the same metals described above (Japanese patent provisional publication No. 60-77357) In these methods, however, a large amount of covering metal is needed when occluding and releasing hydrogen constantly for a long period. Therefore, the amount of hydrogen-storage-alloy becomes relatively small. As a result, capacitance density per unit volume decreases, so that this is a disadvantage in making the battery having high capacitance.
Concerning (2), it is necessary to guarantee the safty of the battery from the standpoint of the user. Therefore, it is necessary to provide with safety valve which releases the gas generated in the battery out of the battery when inner pressure rises above a given value, in order that the battery is not damaged and does not explode in case of the abnormal increment of the inner pressure. However, the actuation of the safety valve causes the electrolyte to be released out of the battery, so that the reduction of the electrolyte causes batteries properties to deteriorate. Therefore, an effectual method for the improvement of battery properties, especially cycling life-time, is to avoid these increments of inner pressure. As a result, materials which do not generate hydrogen on charging have been selected by using materials which have a low equilibrium pressure of hydrogen (Japanese patent provisional publication No. 59-181459 or No. 61-47075).
Concerning (3), various methods have been proposed so that oxygen gas generated when overcharging is reacted effectually so as to decrease the inner pressure of the battery. As described above, the increment of the inner pressure caused by the generation of oxygen gas is one of the reasons for actuating the safety valve. Therefore, it is necessary to change oxygen generated to water rapidly so as to prevent the increment changes in the inner pressure of the battery. Moreover, there is another important problem as the alloy may be oxidized by oxygen gas generated. Hydrogen is or stored in the inner portion of the alloy and an electrical potential itself does not carry out oxidation. However, when repeating charge and discharge, oxidation progresses gradually fr om the surface of the alloy, which surface is in contact with oxygen, to the inner portion of the alloy. As a result, electron conduction properties decrease. Moreover, the amount of hydrogen to be occluded and to be released decreases, and the decrease of a discharge voltage and the decrease of the property of the cycling life-time arise. It is necessary to make an electrode which has an excellent corrosion resistance in order to avoid the above-mentioned problems. However, to the present, a sealed metallic oxide-hydrogen storage battery has not been be obtained which solves the above-mentioned problems (1) to (3) and has high capacitance and long life-time. Moreover, it is well known that in the sealed nickel-hydrogen storage battery, self-discharge is large. An improvement of the self-discharge must be carried out in order that this battery is applied in a practical manner. Therefore, the self-discharge of this battery must be improved at least to the level of the sealed nickel-cadmium storage battery.
Nowadays, in ordinary use, a nonwoven fabric made of a polyamide is utilized as a separator in the sealed nickel-cadmium storage battery. Moreover, a separator wherein surfactants are added to the nonwoven fabric of a polyolefin to improve hydrophilic properties is used for some batteries for applications in high temperature areas. When the nonwoven fabric made of polyamide is applied as the separator to the sealed nickel-hydrogen storage battery, self-discharge becomes a substantial problem. This is because the surface of the negative electrode has a very high activity and strong reducing power in comparison with the cadmium electrode. Another reason is that polyamide is decomposed by charging and discharging, and resulting oxidants and reductants in the ionic state or polar ion coexist in the electrolyte. Thereafter, reactions wherein the materials reduced at the negative electrode are oxidized at the positive electrode are repeated, i.e. NO.sub.3 - and NO.sub.2 -, so that the self-discharge increases. When a separator is used wherein separator surfactants are added to the nonwoven fabric of the polyolefin, charge and discharge cause the property of the surfactants to charge so that the battery property is harmfully affected with the hydrophilic property being decreased. Therefore, a separator is required which is chemically stable to charge or discharge, and has a strong hydrophilic property to the electrolyte.